Refinery planning and feedstock allocation continues to be a very complex problem which must be addressed by petroleum refiners. Uncertainty in feedstock availability, price, and quality has driven the industry to seek flexible primary processing units such as the Fluid Catalytic Cracker (FCC). These have been favored because of their ability to be designed for various operations including maximum distillate, maximum gasoline, and maximum olefins production over a broad spectrum of feedstocks.
Further, many refiners wish to design for a broad slate of feedstocks in order to exploit spot purchases of distressed feedstocks. Feeds of economic opportunity are often heavy and require a specialized FCC to provide a profitable product slate. The optimum selection of feedstocks and the prediction of product yields will be shown to require more complex characterization than simple macroscopic properties such as API (American Petroleum Institute) gravity, carbon residue (Conradson Carbon or Ramsbottom), hydrogen content, etc. Proper consideration must also be given to the processing of paraffinic compounds in the presence of highly contaminated feedstocks with respect to catalytic cracking selectivity and economics of feedstock blends.
To understand the specific issues involved in the FCC processing of paraffinic, high CCR feedstocks consideration should be given to the chemical nature of FCC feeds. Petroleum is primarily a mixture of hydrocarbons together with lesser quantities of other compounds containing sulfur, nitrogen, oxygen and certain metallic elements such as nickel and vanadium. The fractions normally employed as feedstocks to FCC are the materials boiling above about 650.degree. F. These fractions are very complex mixtures, however, for convenience, the United States Bureau of Mines has developed a classification system under which the hydrocarbon portions have been characterized as "paraffinic", naphthenic or asphaltic. Within the vacuum gas oil range (approximately 760.degree. F. boiling point) the stocks are characterized as follows:
Paraffinic.gtoreq.30.degree. API approximately K.gtoreq.12.2 PA1 Intermediate 20.degree.-30.degree. API approximately PA1 K=11.5-12.2 PA1 Naphthenic.ltoreq.20.degree. API approximately K.ltoreq.11.4 PA1 1) A group (designated by the "+" symbol) has API/CCR relationships similar to Light Arabian and it can be inferred that the VGO portion of this feed would be characterized as intermediate (K.about.11.9-12). PA1 2) A group (designated by the ".cndot." symbol) has API/CCR relationships indicating that the VGO is somewhat more paraffinic than that found in Shengli crude with K .about.12.2-12.3. PA1 3) A considerably more paraffinic group (designated by the ".quadrature." symbol) is similar to Minas or Taching and the VGO fraction may have a K as high as 12.4.
where K=characterization factor =(T).sup.1/3 /G when T=mean average boiling point degree Rankine and G=specific gravity at 60.degree. F.
Vacuum gas oils derived from various crude oils exhibit a broad range of variation when measured against these criteria. As the following tabulation illustrates:
TABLE I __________________________________________________________________________ VGO Properties Boiling Gravity Crude Origin Range .degree.F. .degree.API K Description __________________________________________________________________________ Arabian Light Saudi Arabia 650-1050 22.9 11.9 Intermediate Kuwait Kuwait 680-1000 21.4 11.8 Intermediate Brent North Sea 660-1020 26.1 12.1 Intermediate Brega Libya 650-1050 27.7 12.3 Paraffinic Cirita Indonesia 650-1050 34.7 12.8 Paraffinic Shengli China 660-1050 26.5 12.2 Paraffinic Teching China 635-930 34.0 12.4 Paraffinic Isthmus Mexico 650-1000 19.7 11.6 Intermediate Bombay High India 700-1020 29.9 12.5 Paraffinic West Texas Light United States 600-1000 29 12.2 Paraffinic East Texas United States 600-1000 27 12.1 Intermediate Oklahoma United States 490-945 31.5 12.1 Intermediate __________________________________________________________________________
The range of feedstock compositions can further be illustrated by FIG. 6. This data shows the paraffin content of various vacuum gas oils as ranging from 28% (Light Arab) to over 60% (Bombay High). The following Table II is illustrative with respect to atmospheric residual oils (vacuum gas oil plus vacuum bottoms). Assay and mass spectrographic data are presented for Light Arab and Minas atmospheric residues as well as hydrotreated Middle East atmospheric residue. The major differences between the virgin Light Arab and Minas stocks are first in paraffin content and second in the higher level of monoaromatics, in the case of Light Arab. The hydrotreated stock shows that, although after hydrotreating the Middle East stock has an API gravity and CCR similar to Minas, its composition shows that its structure still affects its origin by being similar to Light Arab. The changes are essentially due to boiling range shifts which occur in hydroprocessing.
TABLE II ______________________________________ COMPARISON OF ATMOSPHERIC RESIDUE Light Arab Minas H/T Middle East ATB ATB ATB ______________________________________ Gravity, .degree.API 17.3 26.7 25.1 CCR, wt % 9.8 4.9 3.0 Hydrogen, wt % 12.06 13.3 12.5 Mass Spectrographic Analysis Paraffins 20.6 34.5 25.0 Cycloparaffins 40.1 39.0 36.5 Total Paraffins 60.7 73.5 61.5 Alkyl Benzenes 8.3 2.3 9.8 Benzo-Cyclo Paraffins 6.9 2.9 8.8 Total Mono Aromatics 15.2 5.2 18.6 Diaromatics 10.6 8.1 7.3 Triaromatics & Hur 13.5 13.2 12.6 Total Cord 24.1 21.3 19.9 Aromatics & Hur Total 100.0 100.0 100.0 ______________________________________
Several investigators have studied the relative reaction rates of the various hydrocarbon compounds under catalytic cracking conditions and have developed information useful information to an understanding of our observations and invention.
FIG. 7 shows the FCC conversion of various classes of compounds as a function of severity. This work was done by using amorphous catalyst containing no zeolites. The low reaction rate for normal paraffins on this type of catalyst is quite apparent. At a severity of 1.0, there is still approximately 70% unconverted 430.degree. F.+material as compared with 30% or less for the cycloparaffins and monocycloaromatics.
FIG. 8 tabulates FCC reaction rate constants for five different hydrocarbons ranging from normal paraffins through condensed cycloparaffins. For the amorphus catalyst used (SiO.sub.2 -Al.sub.2 O.sub.3) the rate constants corroborate the ranking shown in FIG. 7. On the other hand, the data shown for a molecular sieve catalyst (REHX) shows first, a much higher reaction rate constant for normal paraffin than in the case of amorphous catalyst and second, a decreased relative reaction rate of condensed cycloparaffins relative to normal paraffins over this type of catalyst. This latter phenomenon is attributed to the greater difficulty for the condensed molecules to enter the zeolite pore structure as compared with the more linear molecules associated with normal paraffins.
Combination fluidized catalytic cracking (FCC)-regeneration processes wherein hydrocarbon feedstocks are contacted with a continuously regenerated freely moving finely divided particulate catalyst material under conditions promoting conversion into such useful products as olefins, fuel oils, gasoline and gasoline blending stocks are well known. Typical modern FCC units employ a riser reactor comprising a vertical cylindrical reactor in which regenerated feedstock are introduced at the bottom, travel up the riser, exit at the top and the catalyst is separated from the hydrocarbon after being in contact for a period of time from about 1-5 seconds.
FCC processes for the conversion of high boiling portions of crude oils comprising heavy vacuum gas oils, reduced crude oils, vacuum resids, atmospheric tower bottoms, topped crudes or simply heavy hydrocarbons and the like have been of much interest in recent years especially as demand has exceeded the availability of more easily cracked light hydrocarbon feedstocks. The cracking of such heavy hydrocarbon feedstocks, many of which are rich in asphaltenes (as evidenced by high Conradson Carbon), results in the deposition of relatively large amounts of coke on the catalyst during cracking. The coke produced by the asphaltenes typically deposit on the catalyst in the early stage of the reaction creating a condition where the cracking catalyst is contaminated by significant levels of coke during the entire reaction system.
A major problem associated with processing residual oil feedstocks, particularly those with high paraffins contents, is this higher tendency to deposit coke per unit mass of catalyst in the reactor riser, particularly at the early stages. This effect is indicated by delta coke which is measured by the difference in the weight percent coke on the catalyst before and after regeneration.
In the case of gas oil feedstocks having a negligible asphaltene content, the delta coke will increase due to coke produced during the catalytic cracking reactions from a negligible value to a value of from about 0.5 to 0.9 as the catalyst travels through the reactor. When processing heavier feedstocks with an appreciable asphaltene content, however, a significant delta coke value will exist immediately at the point of feed vaporization due to the inability to vaporize the heavy asphaltene molecules. In the reactor environment any unvaporized material will undergo thermal degradation which can be expected to yield a certain quantity of unvaporizable heavy hydrocarbon that will deposit on the catalyst. Typically, for example, a feed having a Conradson Carbon level of 5 wt % in which catalyst is circulating at a weight ratio of 5-7 parts catalyst to 1 part hydrocarbon will have an initial delta coke level of 0.4-0.8 and a final delta coke level of 0.8 to 1.3 or higher.
The value of delta coke indicates the degree of fouling the catalyst experiences in the reactor. A fouled catalyst has many of its zeolitic active sites blocked and only a portion of its matrix sites available thereby reducing its cracking activity and selectivity to desired products.
The prime reason for the higher delta coke values observed while processing residual oils is the presence of heavy asphaltene coke producing molecules in the feedstock. The concentration of these molecules is indicated by the value of Conradson Carbon Residue (CCR) associated with the feedstock. Hence, feedstocks with high CCR content will tend to produce high initial delta coke values. The bulk of the feed CCR is associated with the fraction boiling above 1050.degree. F. and therefore, depending upon the size of this fraction, the process parameters for catalytically cracking the feedstock may change significantly from that employed for a typical gas oil.
Challenges with resid processing required new concepts to overcome the many problems associated with the heaviness of the feedstocks, including difficulties in atomizing and vaporizing resids, in reducing high coke yields in then conventional gas oil cracking systems, and in handling extensive heat removal problems due to the high coke yields. Proper catalyst selection was also found to be vital to control and minimize catalyst delta coke (coke yield/catalyst/oil ratio) which is recognized to be an essential catalyst effectiveness parameter.
At present, there are several processes available for fluidized catalytic cracking of such heavy hydrocarbon feedstocks which are known in the art. In such processes, a combination fluidized catalytic cracking-regeneration operation is provided.
Unique catalyst regeneration systems including single or two-stage regeneration systems with partial or full CO combustion are employed to provide the heat removal required when processing high CCR feeds. Also, catalyst coolers have been used to compensate for the high coke level of the catalyst being regenerated.
The hot regenerated catalyst is then employed in the high temperature reaction system to achieve highly selective catalytic cracking for conversion of both high and low boiling components contained in heavy hydrocarbon feeds.
The amount of carbon on the catalyst increases along the reaction path, reducing the number of active sites which can be used for cracking. With high CCR feeds, the coke make rapidly fouls the catalyst, reducing activity immediately upon feed injection. Although the reduced activity may not pose a serious problem to reaction of certain heavy feeds, the problem becomes more acute when the feedstock comprises a high CCR component and a paraffin component, either as separate components of one feed or a blend of multiple feeds.
The blocking of active sites is detrimental because it prevents the cracking of otherwise ideal feed components in an efficient and highly selective manner. This is especially evident when the feedstock contains a significant portion of straight chain paraffins. These paraffins have a high potential to convert to gasoline and lighter material but, as earlier explained, proceeds at a relatively low cracking rate. In the presence of a fouled catalyst and at normal reaction times these molecules do not convert to their full potential resulting in substandard product yields. This problem has little impact in gas oil cracking, but for residual oil cracking the problem is greatly intensified due to the significantly increased delta coke levels.
To illustrate this phenomenon data are presented below on several plant operations.
Plant A
This plant processes a wide variety of residual feedstocks containing gas oils which can be characterized as ranging from intermediate to paraffinic. Operations are typically on feeds having Conradson Carbon levels in the range of 2-5 wt %. Although it is difficult to develop a meaningful value of K for residual oils due to the inability to determine a realistic average boiling point, an approach to feedstock characterization can be developed by use of a gravity/Conradson Carbon relationship as a basis for analogy to known crudes. In FIG. 9, we have plotted three lines which characterize Arabian Light atmospheric residue/VGO in one case and similarly for Shengli and Taching in the others. These lines are developed by connecting the data points of the vacuum gas oil and the atmospheric residue. This gives a basis for selecting operating data based upon the similarity of feedstocks employed to typical residue containing intermediate and paraffinic gas oils. Referring to Table I, Light Arabian VGO has a K of 11.9, Shengli a value of 12.2 and Taching a value of 12.4.
Using this plot as a basis, a selection of data of similar bases was made from the operations of Plant A. FIG. 9 shows three groups of data:
In order to evaluate the conversion efficiency of an FCC operation, a useful parameter is the API gravity of the decant oil or fractionator bottoms streams. This stream essentially consists of the unconverted material boiling above the initial boiling point of the feedstock. Where this value is low (+1 or lower, down to negative values), the conversion of the bulk of the material contained in the feed which is capable of conversion has been converted. FIG. 10 presents data on the decant oil API as a function of delta coke for the three groups of data described above.
In the case of the data for the intermediate feed ("+" points), it is apparent that there is little influence of the delta coke level on the API gravity of the decant oil. However, the influence of delta coke on decant oil gravity is quite pronounced in the case of the data similar to Shengli (".cndot." points) and even more so for the most paraffinic feed (".quadrature." point).
Plant B
Plant B operates on a Mid Continent United States crude and FCC feed data for this unit is plotted on FIG. 9 with "B" symbols. These feeds, while lighter, are similar in relative character to the Plant A feeds which were moderately paraffinic (".cndot." symbol). When the Plant B data are then plotted in FIG. 10, they also show essentially the same delta coke/decant oil gravity relationship as the Plant A data.
Plant C
Plant C processes a fairly paraffinic feed (see point "C" on FIG. 9) and during an eight day period with generally constant feed quality varied feed preheat in operations over a range of catalyst-to-oil ratio which resulted in delta coke ranging from 1 to 1.7. FIG. 11 plots the yield of coke and decant oil (at constant temperature) against delta coke and illustrates the impact of delta coke on overall cracking efficiency.
Plant D
Plant D processes a hydrotreated Middle East residue (as shown in Table II). While on FIG. 9 this feed plots as if it were paraffinic, it was pointed out previously that the composition is closer to an intermediate feed. This is borne out by its operating data (point "D" on FIG. 10) which shows a low decant oil gravity (-2.degree. API) at a high delta coke (1.3). This further illustrates that the paraffin content of the feed is the critical variable.
To achieve the desired product yields under normal reaction conditions, feeds comprising a high Concarbon component and hydrogen rich paraffins require operations designed to achieve a low delta coke, to provide the catalyst activity necessary to crack the paraffins, due to the slow reaction rate of paraffins. This is important since underconversion of the paraffins results in high decant oil yields with high API gravity values. The underconversion of the paraffin component is believed to occur at delta coke levels which exceed about 0.8 to 1.0 (with lower delta coke levels required when paraffin content exceeds 30-35%). This delta coke is created by both feed contaminants and as a normal consequence of the cracking reaction of the feedstocks.
To fully crack feedstocks in this situation, the paraffins must be cracked over a cleaner catalyst, that is, at lower delta coke levels. The known approach is to use a catalyst cooling device and to increase the catalyst-to-oil ratio and therefore lower delta coke. This, however, is not always effective since the delta coke may not be sufficiently reduced or the higher catalyst/oil ratio may overcrack some portions of the products. Further, the higher cat/oil ratio is inefficient in that more catalyst must be passed through the regeneration system resulting in a higher unused coke yield and reduced yields of valuable products.
A number of references relate to the processing of feedstocks having components favoring differing conditions for optimization. A method for optimizing cracking selectivity from relatively lower and higher boiling feeds is described in U.S. Pat. No. 3,617,496. In such a process, cracking selectivity to gasoline production is improved by fractionating the feed hydrocarbon into relatively lower and higher molecular weight fractions capable of being cracked to gasoline and charging said fractions to separate riser reactors. In this manner, the relatively light and heavy hydrocarbon feed fractions are cracked in separate risers in the absence of each other, permitting the operation of the lighter hydrocarbon riser under conditions favoring gasoline selectivity, e.g. eliminating heavy carbon laydown, convenient control of hydrocarbon feed residence times, and convenient control of the weight ratio of catalyst to hydrocarbon feed, thereby affecting variations in individual reactor temperatures.
Another example is seen in U.S. Pat. No. 5,009,769 which describes sending naphtas, boiling below about 450.degree. F., to a first riser and gas oils and residual oils to a second riser.
Other processes which similarly employ the use of two or more separate riser reactors to crack dissimilar hydrocarbon feeds are described, for example, in U.S. Pat. No. 3,993,556 (cracking heavy and light gas oils in separate risers to obtain improved yields of naphtha at higher octane ratings); U.S. Pat. No. 3,928,172 (cracking a gas oil boiling range feed and heavy naphtha and/or virgin naphtha fraction in separate cracking zones to recover high volatility gasoline, high octane blending stock, light olefins for alkylation reactions and the like); U.S. Pat. No. 3,894,935 (catalytic cracking of heavy hydrocarbons, e.g. gas oil, residual material and the like, and a C.sub.3 -C.sub.4 rich faction in separate conversion zones); U.S. Pat. No. 3,801,493 (cracking virgin gas oil, topped crude and the like, and slack wax in separate risers to recover, inter alia, a light cycle gas oil fraction for use in furnace oil and a high octane naphtha fraction suitable for use in motor fuel, respectively); U.S. Pat. No. 3,751,359 (cracking virgin gas oil and intermediate cycle gas oil recycle in separate respective feed and recycle risers); U.S. Pat. No. 3,448,037 (wherein a virgin gas oil and a cracked cycle gas oil, e.g. intermediate cycle gas oil, are individually cracked through separate elongated reaction zones to recover higher gasoline products); U.S. Pat. No. 3,424,672 (cracking topped crude and low octane light reformed gasoline in separate risers to increase gasoline boiling range product); and U.S. Pat. No. 2,900,325 (cracking a heavy gas oil, e.g. gas oils, residual oils and the like, in a first reaction zone, and cracking the same feed or a different feed, e.g. a cycle oil, in a second reaction zone operated under different conditions to produce high octane gasoline).
U.S. Pat. No. 3,791,962 segregates feedstock for feed into separate risers on the basis of an aromatic index and regeneration of the fouled catalyst from each riser in differing initial environments, dealing with the increased coke make of heavier components. In dealing with various coke makes, U.S. Pat. No. 3,791,962 also suggests that temperature affects the yield of carbon.
The prior art, however, does not deal with the issue of difficulty of conversion of paraffinic feeds over contaminated catalysts and, in particular, does not deal with fluidized catalytic cracking of a feedstock containing a significant resid oil fraction (i.e. over 10 vol. %) and a paraffin rich fraction in such a manner as to overcome the unexpected detrimental effects of the combination when each fraction can be optimally processed conventionally.